The present invention relates to control systems for industrial processes and more particularly to hybrid digital/analog control systems for electric power plants.
In the operation of industrial processes and particularly electric power plants, the monitoring and control system commonly includes both an analog control system and a digital computer monitoring and/or control system. Many plant analog signals and many plant contact closure inputs are used by both systems, and many output signals from each system are used by the other system or commonly used to operate a controlled device or a monitoring device. Therefore, an interface between the analog and digital system is needed.
Traditionally, the analog control systems and the digital computer systems for a given installation are designed and manufactured by independent vendors and are interfaced for the first time at the installation site through costly signal conditioning hardware in addition to requiring duplicate process analog inputs and contact inputs for each of the systems. This means an overall complex system at higher initial equipment and installation and maintenance costs to the ultimate user.
In the power plant business, consultants and final users have traditionally considered the analog controls and the digital computer separately with stand alone specifications for each of the systems. Interface problems have resulted in startup delays and non-utilization of the full capability of the individual systems.
The purchase of separate transducers for each system has caused prices to be much higher than is necessary, due to the cost of the sensors themselves, installation costs, cabling costs, cabinet termination areas being duplicated and maintenance of the duplicated sensors.
In the prior United States patent art, Vercelotti U.S. Pat. No. 3,440,613 in FIG. 2 and Harple U.S. Pat. No. 3,351,911 show interface circuitry per se; Craft U.S. Pat. No. 3,818,447 discloses level conversion, and patents such as Stafford U.S. Pat. No. 3,828,325 disclose digital interfaces. None of the known prior art is directed to improvements in the described state of the digital/analog interface art pertaining to the total system level. However, improvements are needed in the structure of such interfaces for manufacturing economy and user convenience purposes. No representation is made that the prior art cited herein is the best prior art nor that other interpretations cannot be placed on it.